This invention relates to a continuous process for thermal splitting of carbamic acid esters.
The thermal splitting of carbamic acid esters has been known for some time. The work of A. W. Hoffmann (Berichte der Deutschen Chemischen Gesellschaft, Vol. 1870, pages 653 et seq) and M. Metayer (Bull. Soc. Chim. France, Vol. 1951, pages 802 et seq), has shown that such thermal splitting is reversible, i.e. when the hot reaction mixtures are cooled, the isocyanatee recombine with alcohols and carbamic acid esters are reformed. Special measures must therefore be taken to ensure that the isocyanates and alcohols formed in the thermal splitting of carbamic acid esters can be separately obtained.
U.S. Pat. No. 2,409,712 describes a process by which monofunctional isocyanates may be obtained in batches in moderate yields by heating the corresponding carbamic acid esters in batches to temperatures of from 150.degree. to 450.degree. C. The cleavage products may then be separated by rapid distillation or by introduction into a solvent system which selectively dissolves the isocyanate and the alcohol (for example a mixture of cyclohexane and water). Although this disclosed process is well suited for batch production of monofunctional isocyanates on a laboratory scale, it is commercially impractical due to the fact that it can be carried out only in batches and only moderate yields are obtained. This disclosure does not, however, teach anything with respect to suppression of the formation of secondary products which form during the thermal splitting of carbamic acid esters. Nor does this disclosure teach how to eliminate or at least reduce the adverse effects which such secondary products have on the commercial operation of the splitting process.
Studies conducted by H. Schiff (Berichte der Deutschen Chemischen Gesellschaft, Vol. 1870, pages 649 et seq) and by E. Dyer and G. C. Wright (J. Amer. Chem. Soc., Vol. 81, 1959, pages 2138 et seq) show that, under thermal load, carbamic acid esters can undergo complete or partial, irreversible decomposition to form different products. These possible products include substituted ureas, biurets, carbodiimides, isocyanurates, secondary amines, olefins and/or carbon dioxide.
Various processes have been developed with a view to suppressing the formation of undesirable secondary products in the thermal splitting of carbamic acid esters. One obvious possibility is to minimize the thermal load imposed during the splitting process. If this alternative is used, the thermal splitting process generally must be carried out in the presence of a catalyst because otherwise the volume/time yields would be too low to be practical.
U.S. Pats. Nos. 2,713,591; 2,692,275; 2,727,020 and 4,294,774 and Japanese Patent Application No. 54-88201 (1979) describe processes for the production of isocyanates by thermal splitting of carbamic acid esters in the presence of basic catalysts. However, such basic catalysts lead to increased, irreversible decomposition reactions of carbamic acid esters and isocyanates (cf. for example J. Appl. Polym. Sci., Vol. 16, 1972, page 1213). Accordingly, processes using basic catalysts only give acceptable yields of isocyanate when the carbamic acid esters used are protected against decomposition by appropriate substituents.
Another possible method for suppressing secondary reactions in the thermal splitting of carbamic acid esters is to dilute the carbamic acid esters and/or the cleavage products with inert diluents. U.S. Pat. No. 3,919,279, German Offenlegungsschrift No. 2,635,490 and Japanese Patent Application Nos. 54-39002 (1979) and 54-88222 (1979) describe processes in which the thermal splitting of carbamic acid esters is carried out in inert solvents, optionally in the presence of certain catalysts. In addition to inert solvents, carrier gases, optionally in the form of evaporated low-boiling solvents, are used in the processes described in German Auslegeschriften Nos. 2,421,503 and 2,526,193. However, the use of solvents in the thermal splitting of carbamic acid esters gives rise to considerable difficulties. The solvent used must be stable and inert to isocyanates under the thermolysis conditions. The solvent must also be readily miscible with the carbamic acid esters to be split and its vapor pressure at the temperatures applied must be low enough that it will remain substantially in the liquid phase during the thermolysis process. The choice of possible solvents is seriously limited by these requirements. Suitable, inexpensive solvents are particularly difficult to find in cases where the carbamic acid esters to be split have high molecular weights. In addition, the use of solvents reduces the volume/time yields of isocyanates. Further, in cases where high-boiling solvents are used, it is difficult to distill off the pure components of the liquid reaction mixtures (residues of isocyanate, carbamic acid ester and solvent) from the residue, as proposed for example in German Auslegeschrift No. 2,530,001. In any event, considerable extra effort is involved in working up and storing inert solvents.
The above-mentioned processes for the thermal splitting of carbamic acid esters depend on the use of diluting solvents particularly where polyfunctional carbamic acid esters are used for splitting. If such solvents were not used, the isocyanato-urethanes inevitably accumulating in the reaction mixture would continue reacting to a considerable extent to form undesirable secondary products.
The thermal splitting of monofunctional carbamic acid esters may, however, be carried out in the absence of diluting solvents without serious losses of yield. This is shown, for example, by the process for the thermal splitting of monofunctional carbamic acid alkyl esters described in an earlier, but unpublished German Patent Application No. P 30 47 898.9 (European Patent Application No. 81 110 204.5). This process is, however, disadvantageous in that relatively long residence times for the carbamic acid alkyl esters to be split are necessary.
Finally, splitting processes in which carbamic acid esters are split at high temperatures (400.degree. to 600.degree. C. or 350.degree. to 550.degree. C.) in the gas phase are described in U.S. Pats. No. 3,734,941 and 3,870,739. In these processes, it is important that the residence time of the gases in the high-temperature zone remain as short as possible because otherwise the carbamic acid esters and/or the cleavage products undergo serious decomposition under the effect of the high thermal load, despite dilution by the gas phase. However, such short residence times can result in yields of isocyanates which are undesirably small. Further, these processes require considerable technical outlay because gases are difficult to heat and cool over short periods of time due to their poor thermal conductivity.